1. Field of the Invention
The present invention relates to process for producing a pigment complex and a pigment-containing composition. The present invention particularly relates to a process for producing a pigment complex and a pigment-containing composition which possess good dispersion stability over a long period of time, and which have a small and uniform particle size.
2. Related Background Art
Inkjet recording systems, as a direct recording system, is highly characterized in that such systems are compact and have low power consumption. In addition, due to nozzle miniaturization, improvements in image quality and high-speed printing are rapidly progressing. One example of inkjet technology is a method which forms an image on a recording medium by heating ink supplied from an ink tank with a heater in the nozzle to vaporize and foam the ink, and thereby ejected. Another example is a method which ejects ink from a nozzle by vibrating a piezo element.
Since the ink used in such methods is usually an aqueous dye solution, in some cases phenomena are manifested such as bleeding, which occurs when colors are superimposed on each other, or feathering in the fiber direction of the paper at a recording location on the recording medium.
The use of a pigment dispersion ink in order to improve these matters is being investigated. For example, Japanese Patent Application Laid-Open No. H4-28776 discloses an ink which uses a pigment in place of a dye. U.S. Pat. No. 5,085,698 discloses an ink in which a pigment has been dispersion-stabilized using a polymer dispersing agent, and it was confirmed that light resistance, water resistance and fixing properties showed a substantial improvement compared with the case of a dye. The pigments used in these inks are obtained by a process which mechanically finely pulverizes an agglomeration of an ordinary commercially available pigment using a ball mill or other such disperser. For this reason, the average particle size of such fine pulverized pigments is usually greater than about 100 nm (0.1 μm), and further, the particle size distribution is comparatively broad.
In contrast, Japanese Patent Application Laid-Open Nos. 2002-285032 and 2002-138229 disclose methods for producing a pigment having a particle size of 100 nm or less through the crystallization of a pigment dissolved in a supercritical fluid.
As stated above, since pigments possess advantageous effects compared with dyes, their use has been widely investigated. In recent years, finer, more colorful and stronger tinting pigment inks are being demanded. It is thought that to realize such finer inks having excellent purity that it would be preferable to employ small and uniform pigment particles as the ink composition. However, it is extremely difficult to reduce the size of the obtained primary particles in the above-described pigment fine pulverizing process carried out using a disperser. Therefore, since the particle size distribution of the pulverized particles is broad, there has thus been the problem that large pigment particles would block the inkjet nozzles. In view of such a problem, a pigment is desired which doesn't contain large particle size pigment particles, and which has a small and uniform particle size distribution.
With the above-described fine pulverization using a disperser, there is also a drawback that a process taking up time and money is required. In addition, because pigments are essentially insoluble in water, even the above-described small-size pigments which employed a supercritical fluid have in fact in-sufficient dispersion stability able to withstand actual use.